Implement WheelEvent Interface

Note: The WheelEvent interface supports rotation in all 3 spatial
dimensions. This implementation only supports two due to limitations
in the Glutin compositor.

The wheelevent interface is a dom interface that triggers for any
attached device that can rotate in one or more spatial dimensions.
Traditionally this is the mouse wheel though other devices could be
used as well. E.g. the trackball on a trackball mouse.

components/script_traits/lib.rs

@@ -462,6 +462,30 @@ pub enum MouseEventType {
     MouseUp,
 }
 
+/// Mode to measure WheelDelta floats in
+#[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]
+pub enum WheelMode {
+    /// Delta values are specified in pixels
+    DeltaPixel = 0x00,
+    /// Delta values are specified in lines
+    DeltaLine = 0x01,
+    /// Delta values are specified in pages
+    DeltaPage = 0x02,
+}
+
+/// The Wheel event deltas in every direction
+#[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]
+pub struct WheelDelta {
+    /// Delta in the left/right direction
+    pub x: f64,
+    /// Delta in the up/down direction
+    pub y: f64,
+    /// Delta in the direction going into/out of the screen
+    pub z: f64,
+    /// Mode to measure the floats in
+    pub mode: WheelMode,
+}
+
 /// Events from the compositor that the script thread needs to know about
 #[derive(Debug, Deserialize, Serialize)]
 pub enum CompositorEvent {
@@ -491,6 +515,8 @@ pub enum CompositorEvent {
         Point2D<f32>,
         Option<UntrustedNodeAddress>,
     ),
+    /// A wheel event was generated with a delta in the X, Y, and/or Z directions
+    WheelEvent(WheelDelta, Point2D<f32>, Option<UntrustedNodeAddress>),
     /// A key was pressed.
     KeyboardEvent(KeyboardEvent),
     /// An event from the IME is dispatched.